Posted by DDT has a checkered history, to be sure. Many of us remember walking through clouds of it in our childhoods, as it was sprayed willy-nilly for nuisance mosquitoes. The discovery that it was persistent in the environment (didn’t break down) and harmed birds by thinning their egg shells (Rachel Carson’s “silent spring”) eventually led to its withdrawal from use. It is banned in the US, although it is not banned worldwide and is still used for vital public health purposes. Most of the actual uses during its heyday were for economic or aesthetic purposes with no public health rationale. Its disappearance for these uses has been a good thing. Through all the controversy (most of it stimulated by agribusiness who liked DDT’s effectiveness and cheapness), there has been uncertainty about its effect on people. In the last few years various epidemiological studies of breast cancer failed to find a connection with that dread disease. Now a new study does find one and suggests why the others haven’t. In breast cancer, as in the rest of life, timing is everything:
Women heavily exposed to the pesticide DDT during childhood are five times as likely to develop breast cancer, a new scientific study suggests.
For decades, scientists have tried to determine whether there is a connection between breast cancer and DDT, the most widely used insecticide in history. The UC Berkeley research, based on a small number of Bay Area women, tested a theory that the person’s age during exposure was critical, and provided the first evidence of a substantial effect on breast cancer.
“There was very broad exposure to this pesticide, and with this study, we have evidence that women exposed when young were the most affected,” said Barbara A. Cohn, director of UC Berkeley’s Child Health and Development Studies, who led the study of 129 women. “If this finding holds up, those who were young and more highly exposed could be the women at greatest risk.”
Women born between 1945 and 1965 were most likely to have been heavily exposed as children to DDT, which was sprayed throughout the United States to kill mosquitoes and other insects. DDT use began in 1945, peaked in 1959 and was banned nationwide in 1972 because it was building up in the environment. (LA Times)
The unique feature of this study is the availability of stored serum samples from women who gave birth to children in the 1959 – 1967 period, a time when DDT was still used in high volume and before any of them developed breast cancer. In considering the group who developed breast cancer later (median time to diagnosis was 17 years post sample), those with the highest DDT levels and who were born after 1931 had a five fold elevated breast cancer risk. The 1931 date was chosen because those women would have been 14 years old or less in 1945 when DDT use started in earnest in the US. The data thus suggest that it is the timing of exposure which is critical, not just some later measured value of DDT metabolites in middle aged women, the usual subjects of the DDT breast cancer studies. Early exposures (adolescence and childhood) are the key to DDT risk in these data.
This is one study, albeit a well conducted one by experienced investigators. It is also relatively small, limited by the number of historical stored serum samples. But the effect is large, which makes it less likely we are seeing the consequences of some hidden bias (an unmeasured variable related to breast cancer risk that differs dramatically in the high DDT group versus its comparison). It is possible for such differences to exist, but for such a large effect the unmeasured variable (an uncontrolled confounder) would have to be a powerful risk factor we don’t know about. Possible. Not that likely.
Confirmation and further work are needed — as always: when have scientists not asked for it? But these data are concrete evidence that the caution about human exposure to DDT was warranted and banning its use in the US was of benefit to more than birds.
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